1. Field of the Invention
This invention pertains to tubular connections and more particularly to tubular connections having a metal-to-metal seal forward of the threads on the pin member of the connection.
2. Description of the Prior Art
Tubular connections comprise a pin or male member and a box or female member. In some cases, two lengths of pipe or other tubular goods are joined pin end to pin end employing a coupling having two box ends. In most cases, however, the pin end of one tubular member is secured directly into the box end of an adjoining tubular member. As referred to herein, a "tubular connection" is understood as meaning a connection involving a pin member and a box member, regardless as to whether that box member is included in a coupling or in a tubular length of product.
Although different connections include a sealing arrangement at different places, a popular location for a metal-to-metal seal location in a tubular connection is in a conical area located forward of the first thread in the pin member of the connection. This area is not within the threads and is usually made between matching surfaces of the pin member and the box member on a surface that has a slight longitudinal taper with respect to the axis of the connection. Usually this taper is on the order of about 7.degree., although very shallow tapers of about 2.degree. and steeper tapers of about 14.degree. are also fairly common.
In the prior art connections, the seal area on the pin member tapers from a point in front of the first thread to the tip end of the pin member. The two mating surfaces come in contact as the connection is torqued so that when it is completely power torqued there is an area range within this overall area that is tightly pressed sealed together. To further describe the areas on the pin member and the box member, there is a short section in front of the most forward thread on the pin member where no sealing contact occurs. The conical metal-to-metal surface then progresses toward the tip or nose or distal end of the pin member. There is a small chamfer between the conical surface and the tip end. Therefore, the entire metal-to-metal sealing area is located between the chamfer and the non-sealing area in front of the first thread. It is assumed for purposes herein that the box member conical surface corresponding to the pin member conical surface is at an identical longitudinal taper. In many cases the box taper is slightly steeper than the taper of the pin member surface; however, this invention relates to such connections as well. For convenience of discussion, the taper of the pin member and the box member will be considered to be at the same angle.
Returning to the pressed seal condition of the connection, an analysis shows that the area where the surfaces are pressed sealed together the hardest occurs in the conical seal area nearest the first thread on the pin member. That is, for identical conical sealing surfaces on the pin member and box member, although the area in sealing contact might appear casually to be the entire surface, only the approximate one-tenth to one-third of the area nearest the first thread will be leak sealed with respect to internal gas pressures above about 20,000 psi. Further with respect to the prior art connection just described, the metal of both the pin member and box member is stressed upon full torquing in a relative narrow concentration band. This is because the tip end tends to flex slightly away from being tightly pressed sealed.
Therefore, it is a feature of the present invention to provide an improved tubular connection with a stiffer pin member in the vicinity of the seal on the forward end of the pin member to increase the integrity of the seal by increasing its bearing surface, thereby also reducing the tendency of the connection to galling.
It is another feature of the present invention to provide an improved tubular connection with a pin member having an extended nose in front of the seal area of the pin member to stiffen the pin member and thereby enhance the connection.